Protective stepping is a common response used for balance recovery. Nevertheless, the mechanisms underlying situations where protective stepping fails are largely unexplored and unknown. We are pioneering a line of work that focuses on the precise causes for failure in protective stepping. We expect our research to pave the way for future interventions based on the specificity principle in physical rehabilitation. We will first demonstrate that limb collapse is the major cause for failure in protective stepping, and that age and gender are critical determinants in successful attempts to regain balance after limb collapse. We will test the hypotheses that limb collapse will occur in all subjects during unexpected slipping in the execution of a daily activity, and that a greater number of older adults (ages 65-80 years) and female adults will not be able to recover from limb collapse than young adults (ages 22-45 years) and male adults. In order to understand why some older subjects are able to recover from limb collapse while other fail to do so, we will next determine the biomechanical causes for limb collapse. We will test the hypotheses that limb collapse is inevitable in some graduate subjects because the supporting knee (instead of hip or ankle) has failed to generate torques to reach threshold value for recovery, and furthermore that this threshold is predictable. In order to find the specific means to strengthen an individual's resistance to limb collapse, we will then examine the neuromuscular mechanisms for recovery from limb collapse. We will test the hypotheses that insufficient extension torques result from a prolonged, reflexively mediated inhibition similar to some component of startle or other related responses, rather than from a failure of feed-forward programming during the critical time-window of center of mass vertical descent. Our long-term goal is to develop rehabilitation programs that will facilitate and fortify elderly individuals' own resistance to catastrophic falls due to limb collapse. The results will be a reduction in both costs of health care and in unnecessary injury that currently lowers the mobility and the quality of life of victims of catastrophic falls.